FIG. 10 is a cross-sectional view of a hetero-junction field effect transistor (HFET) described in Japanese Laid-Open Patent Publication No. 2004-273486 (Patent Document 1).
An HFET 100 depicted in FIG. 10 has a nitride buffer layer 102, a first nitride semiconductor layer 103, and a second nitride semiconductor layer 104 formed in this order on a substrate 101.
The HFET 100 depicted in FIG. 10 also includes a source electrode 105, a gate layer 109, a gate electrode 110, and a drain electrode 106. The source electrode 105, the gate layer 109, and the drain electrode 106 are formed separately from each other on the second nitride semiconductor layer 104. The gate electrode 110 is formed on the gate layer 109. A material of the gate layer 209 is a p-type semiconductor layer.
In the HFET 100 depicted in FIG. 10, a two-dimensional electron gas (2DEG) generated in an interface between the second nitride semiconductor layer 104 and the first nitride semiconductor layer 103 acts as a channel.
The HFET desirably has a normally-off characteristic. As used herein, the “normally-off characteristic” refers to a state in which a thickness of a depletion layer immediately below a gate is sufficiently thick while no electron is present when the gate voltage is 0 V.
When a predetermined voltage is applied between the source electrode 105 and the drain electrode 106, electrons in the channel move from the source electrode 105 to the drain electrode 106. In this case, the “normally-off characteristic” means that no drain current flows when the gate voltage is 0 V, i.e., when no voltage is applied to the gate electrode 110.
A conventional nitride semiconductor device has the gate layer 109 made of a P-type semiconductor disposed as described above so as to acquire the normally-off characteristic. The P-type semiconductor depletes the electrons of the two-dimensional electron gas (2DEG) immediately below the gate layer 109 to acquire the normally-off characteristic.
A nitride semiconductor device disclosed in Japanese Laid-Open Patent Publication No. 2009-076845 (Patent Document 2) uses NiO as the P-type semiconductor.